UFO: Extraterrestrials works a bit like this: if you manage to get plasma weapons from the aliens (which is hard, as it requires capturing a live Commander), they switch to ion weapons. When you get ion weapons, they get the next tier etc. It's an arms race you can never win, unless they run out of tiers.
This model has some good points, as it always makes you discover new stuff, but it's also very crude. Besides, if provokes metagaming: why research stuff if it's not really improving your situation?
If anything, aliens should only get stronger over time, or if the invasion simply is going well (for example, depending on how many countries they have on their side). It'd be harsh, but interesting. I wonder what exactly they could get, though; I'm not in favour of new better weapon tiers, and simply increasing their numbers would be... stretching it.